Metallic articles are fabricated by any of a number of techniques, as may be appropriate for the nature of the metal and the article. In one common approach, metal-containing ores are refined to produce a molten metal, which is thereafter cast. The metal is further refined as necessary to remove or reduce the amounts of undesirable minor elements. The composition of the refined metal may also be modified by the addition of desirable alloying elements. These refining and alloying steps may be performed during the initial melting process or during subsequent remelting. After a metal of the desired composition is produced, it is cast. The cast piece may be used in the as-cast form for some alloy compositions (i.e., cast alloys), or further worked to form the metal to the desired shape for other alloy compositions (i.e., wrought alloys). Instead of casting it to shape, the molten metal may be atomized to produce fine-scale droplets, which then solidify to form fine-scale powders. The powders are thereafter compacted or consolidated to produce useful articles. In any case, further processing such as heat treating, machining, surface coating, and the like may be employed.
Metallic alloys in finely divided form are utilized for a number of processes in order to minimize the scale of solidification segregation (as a result of the droplet size and cooling rate) and/or to provide a convenient material form for transport to, and application on, a substrate surface. During some processes, for example liquid or semi-solid phase deposition and fusion welding processes, the material is melted during or after application. In some other processes, for example cold spray, slurry coating, laser sintering, and vapor-phase deposition, the material is not subsequently melted during the application process. In still other processes, for example brazing and transient liquid phase bonding, the material is melted but for a very brief period of time, typically less than 10 minutes.
These fabrication processes have fundamental limitations. Some metallic alloy compositions cannot be prepared by melting techniques in commercial-scale production due to thermophysical melt incompatibilities between the alloying elements. The melting operation can also result in impurities in the melt, which then reach the final product unless special and costly refining steps are used. In some cases, the introduction of the impurities is not known, and removal is not addressed. The melting operation can leave unmelted inclusions in the metallic final product, such as ceramic particles from the melting crucible, and particles of oxide dross. These inclusions may lead to early failure of the final article due to crack initiation and propagation. The macrostructure and microstructure of the final article are dictated in part by the structure produced upon solidification, and in many instances great efforts are expended to alter the as-cast microstructure to improve mechanical properties. Powder techniques are employed to reduce composition and structure variations inherent in cast and wrought materials across the article, but the powder techniques have their own disadvantages such as incorporation of the atomization gas into the powder particles, and extraneous contamination of powder during processing.
Incremental performance improvements resulting from processing modifications are still possible in a number of areas. However, the present inventors have recognized in the work leading to the present invention that in other instances the basic fabrication approach imposes fundamental performance limitations that cannot be overcome at any reasonable cost. They have recognized a need for a departure from the conventional thinking in fabrication technology which will overcome these fundamental limitations. The present invention fulfills this need, and further provides related advantages.